The present invention relates to a method of driving a liquid crystal optical device, in order to obtain a light-transmission state or a light-shielding state by use of an electro-photo effect of liquid crystal.
In recent times, increasing use has been made of liquid crystal optical devices for obtaining a light-transmission state or a light-shielding state by incorporating them in television sets, computer terminals, office equipment, and the like. A liquid crystal optical device is used in optical printing head of a matrix display device or in an electro-photographic printer incorporated in the above-mentioned devices. The liquid crystal optical device controls the light-transmission (ON) states and the light-shielding (OFF) states of microshutters, so that an image and characters are displayed in the display device, or a pattern such as an image and characters are exposed onto a photosensitive body in the optical printing head. The microshutters are independently ON/OFF controlled, and must be as small a size as possible, so that a large number of them can be arranged on a large screen, in order to improve the reproducibility of an image, when they are used in a display device, or so that an image with high resolution can be obtained when they are used in an the optical printing head of electro-photographic printer. Since a large number of microshutters are driven in order that a picture or an image can be produced with high reproducibility, the liquid crystal optical device must satisfy the following requirements:
(1) a high ratio of intensities of transmitted light and a high ratio of amounts of transmitted light between the ON and OFF states of the microshutter, i.e., high contrast,
(2) a high intensity of transmitted light in the ON state of the microshutter, and
(3) high-speed response.
Several electro-photographic printers have been proposed which may be used as the computer terminals or office equipment described above.
For example, an alphanumeric printer system is disclosed in U.S. Pat. No. 3,824,604, which uses a matrix type liquid crystal device in which a nematic cholesteric compound is sealed between a pair of substrates on which electrode matrices are formed. This printer system incorporates a liquid crystal matrix in which each alphanumeric character is constituted by a 7 (lines).times.5 (rows) block. Fiber optic lines are provided for each block. When the liquid crystal matrix is ON, light is guided through the fiber optic lines, onto a photosensitive drum (selenium drum), whereby alphanumeric characters using the 7.times.5 blocks are printed on the drum. However, in this printer system, since the liquid crystal matrix uses a nematic cholesteric compound and each character has a 7 (lines).times.5 (rows) pixel arrangement, an alphanumeric character and/or image cannot always be accurately reproduced in a desired format The printer system has a rise time of 10 msec and a decay time of 350 msec, i.e., a low operating speed.
An electro-photographic printer which uses a twisted nematic type (to be referred to as a TN type hereinafter) liquid crystal device is disclosed in DE. 2711194-A1 and U.S. Pat. No. 4,297,022. The liquid crystal device of the printer uses a microshutter array in which microshutters corresponding to pixels are aligned in one or two lines. The liquid crystal device uses a TN type liquid crystal material. Therefore, even if the driving voltage, the thickness of the liquid crystal layer, and the like are set to optimal values, the rise time and the decay time of the liquid crystal device can, at most, only be improved to around 2 msec and 20 msec, respectively.
When an electro-photographic printer utilizing the liquid crystal device described above is put into practical use, each microshutter must be of very small size, e.g., 0.1 mm.times.0.1 mm square or smaller, in order for a high-resolution printed image to be obtained. An image printed on one page of a paper sheet is formed of a large number of dots which correspond to light spots controlled by the micro shutters. For this reason, in order that a practical printing speed can be attained, at which about ten A4-size paper sheets can be printed per minute, the microshutters must therefore be driven at very high speed. Therefore, the above-mentioned known electro-photographic printer cannot attain the required, i.e. high, printing speed.
As a first method for solving the above problem, a TN type liquid crystal device having liquid crystal molecules of increased twist angle is driven by a two-frequency addressing scheme, so that a high-speed response can be obtained In this TN type liquid crystal device, a relatively large amount of chiral liquid crystal, for increasing twist power, is added to a liquid crystal compound exhibiting a dielectric dispersion phenomenon. A high-frequency electric field, higher than a crossover frequency of the liquid crystal material, and a low-frequency electric field, lower than the crossover frequency, are selectively applied to the liquid crystal material of the liquid crystal device. The liquid crystal molecule alignment is controlled by the two-frequency addressing scheme, as follows:
An electric field of a high frequency (e.g., 100 kHz) is applied to a liquid crystal, to align the molecular axes of liquid crystal molecules so that they are perpendicular to the applied electric field, and an electric field of a low frequency (e.g., 200 Hz) is applied thereto, to align the liquid crystal molecular axis so that it is parallel to the applied electric field. Since, by using this addressing scheme, the liquid crystal molecules are aligned perpendicular to and parallel to the substrate upon application of the electric fields, the liquid crystal device is capable of high-speed response.
Printers using the above-mentioned TN type liquid crystal device are disclosed in U.S. Pat. No. 4,386,836, DE. No. 3213872-A1, U.S. Pat. No. 4,609,256, and EP No. 0083253.
The liquid crystal device used in these printers can ON/OFF-control the microshutters such that electric fields of high and low frequencies are alternately and repetitively applied every 1 msec.
However, in the TN type liquid crystal device, the liquid crystal molecules must be twist-aligned by the self twist power of liquid crystal molecules. Since the twist alignment causes a delay between each operation of the device the device, therefore, has a limited maximum response speed. More specifically, when, in a positive display (a normally-ON display in which a pair of polarizing plates are arranged so that their polarizing axes are perpendicular to each other), a high-frequency electric field is applied to the liquid crystal material so as to change an OFF state to an ON state, the response speed is delayed by the time required for the twist alignment of the molecules, and thus, a short rise time is difficult to obtain. Since the twist pitch of the liquid crystal molecules is changed, according to the temperature and thickness of the liquid crystal layer, the TN type liquid crystal device possesses poor temperature stability, and thus the thickness of the liquid crystal layer must be precisely controlled. In addition, since the molecules of the TN type liquid crystal material have a large twist angle, light leakage arising from optical rotary dispersion is considerable, and contrast is low.
As a second method for solving the above problem, a number liquid crystal devices utilizes a Guest-Host effect type liquid crystal. In the GH type liquid crystal device, a liquid crystal material driven by a two-frequency addressing scheme, to which a dichroic dye is added, is sealed between substrates. The substrates are subjected to homogeneous aligning treatments in directions parallel to each other. In this method, a liquid crystal composition exhibiting a dielectric dispersion phenomenon is used as the liquid crystal material.
An apparatus for forming an image by use of the above GH type liquid crystal device has already been filed as U.S. Ser. No. 630,957 by several of the present inventors. This apparatus is also filed in the U.K. as GB No. 2144869B.
In the GH type liquid crystal device, dye molecules are aligned so as to be parallel to the substrate, together with liquid crystal molecules, upon application of the high-frequency electric field, and an OFF state is established, since light of a specific wavelength range is absorbed by the dye. When the dye molecules are aligned to be perpendicular to the substrate together with the liquid crystal molecules, they cannot absorb light, thereby establishing an ON state.
In the GH type liquid crystal device, a transition between the ON and OFF states can be realized by homogeneous and homeotropic alignments of the liquid crystal molecules without twist alignment. Therefore, this device can provide a higher response speed than that of the TN type liquid crystal device because of absence of a delay time corresponding to twist alignment, and temperature stability is also improved. The effective amount of transmitted light during the ON operation can be increased, and a high contrast can be obtained during practical operation due to high response speed. As a result, when the GH type liquid crystal device is applied to an optical printing head, liquid crystal optical printers reach a technical level which allows actual mass production.
However, in the liquid crystal optical printer using the GH type liquid crystal device, some problems still remain unsolved. These problems are requirements for a still higher contrast and stabler temperature characteristics. More specifically, the contrast of the GH type liquid crystal device mainly depends on a dichroic ratio of the dye added, a dye concentration, a cell thickness, and the like. For this reason, the dye added to the liquid crystal must satisfy the following requirements:
(1) it must have a high dichroic ratio;
(2) it must have a maximum absorption wavelength range which coincides with a maximum emission wavelength range of emission spectral characteristics of a light source;
(3) it must have a high solubility with respect to a liquid crystal as a "host"; and
(4) it must have a high absorbency.
Since it is difficult to find a dye which can satisfy all the above requirements, an improvement in contrast of the liquid crystal device is limited. Since fluctuation of the dye molecules changes depending on temperature, a transmittance in the OFF state is unstable, and hence, the temperature stability of contrast cannot be improved, resulting in a very narrow operation temperature range.
As a third method for solving the above problem, some devices for controlling ON/OFF state adopt an electrically controlled birefringence type liquid crystal device. In this liquid crystal device, a nematic liquid crystal (Np liquid crystal) having positive dielectric anisotropy .DELTA..epsilon. is sealed between a pair of substrates which are respectively subjected to a homogeneous aligning treatment in directions parallel to each other, and polarizing plates are respectively arranged on the outer surfaces of the pair of substrates. The polarizing axes of these polarizing plates are perpendicular to each other, and intersect the direction of the homogeneous aligning treatment of the substrates at 45.degree..
When an electric field is applied to the liquid crystal device, liquid crystal molecules are homeotropically aligned with respect to the substrates and the polarizing axes of the pair of polarizing plates are perpendicular to each other. Therefore, no light is transmitted therethrough, and an OFF state is established. When the electric field is turned off, the liquid crystal molecules tend to return to a homogeneous alignment state due to their alignment power, and the liquid crystal device transmits light in a tilt alignment state during a transition from the homeotropic alignment state to the homogeneous alignment state, thereby establishing the ON state.
Liquid crystal optical printers using the liquid crystal device are disclosed in U.S. Pat. Nos. 4,569,574, 4,595,259, and Japanese Patent Disclosure (Kokai) Nos. 56-115277, 59-119330, and 60-182421. In the liquid crystal device, since the liquid crystal molecules need only behave between the tilt alignment state and the homeotropic alignment state with respect to the substrates during the ON and OFF operations, their operation angle can be small. In addition, since the liquid crystal molecules need not be twist-aligned, the device can have a considerably higher response speed than the TN type liquid crystal device using an Np liquid crystal. However, in the liquid crystal device, a power for obliquely aligning the liquid crystal molecules is defined only by the alignment power acting between the substrates and the liquid crystal molecules. For this reason, a response speed during the transition from the OFF state to the ON state is still low. The operating speed of the liquid crystal molecules depends on the viscosity of the liquid crystal, and the viscosity depends on a temperature. Therefore, the liquid crystal device has poor temperature stability.
In order to solve the above problem, another technique has been proposed. In this technique, a liquid crystal composition exhibiting a dielectric dispersion phenomenon is used for the above-mentioned electrically controlled birefringence type liquid crystal device, and is turned on/off upon selective application of electric fields of two frequencies, i.e., high and low frequencies.
Liquid crystal optical printers using the liquid crystal device are disclosed in Japanese Patent Disclosure (Kokai) Nos. 58-176620 and 61-87136. In the liquid crystal device used in these printers, the liquid crystal molecules behave upon application of the electric field when the liquid crystal molecules are homeotropically aligned and when they are obliquely aligned Therefore, the device can have a considerably high response speed. In addition, the liquid crystal optical printer using the liquid crystal device has a possibility of high-speed printing.
The printing speed of the liquid crystal optical printer depends on a time during which a charge for one point on a uniformly charged photosensitive drum is discharged by light transmitted through micro shutters provided to the liquid crystal device. When an image for one page of a paper sheet is formed, a large number of dot lines must be written. Therefore, if a time required for writing one dot line is long, it requires a very long time to write an image for one page, and high-speed printing cannot be performed. On a portion corresponding to one dot line on the photosensitive drum, a surface charge is discharged when a product of a radiation time and the intensity of light transmitted through the micro shutter array of the liquid crystal device and radiated on the photosensitive drum reaches a predetermined value. The photosensitive drum senses light depending on the amount of light transmitted through the individual micro shutters For this reason, if the intensity of light transmitted through each microshutter is low, light radiation for a long time per dot line is necessary. The amount of light depends on the response speed of the micro shutters More specifically, if a response speed is low when the micro shutters are switched from the OFF state to the ON state and/or are switched from the ON state to the OFF state, a sufficiently opened ON state or OFF state cannot be established within a predetermined period of time for turning on/off the micro shutter once, and a transmitted light intensity is low. Even though the sufficiently opened ON state or OFF state is established, a duration during which the established state is maintained is short, resulting in a small amount of light.
The response speed changes in accordance with the viscosity of the liquid crystal material and dielectric anisotropy .DELTA..epsilon., and the like. The viscosity and the value of dielectric anisotropy .DELTA..epsilon. have temperature dependency. For this reason, since the response speed changes in accordance with a change in temperature, it has poor temperature stability. More specifically, the viscosity of the liquid crystal material is decreased as the temperature is increased. As a result, the response speed is increased. Crossover frequency fc at which dielectric anisotropy .DELTA..epsilon. in a dielectric dispersion phenomenon of the liquid crystal material becomes zero is increased upon an increase in temperature, and absolute values of dielectric anisotropy .DELTA..epsilon.L in the low-frequency electric field and dielectric anisotropy .DELTA..epsilon.H in the high-frequency electric field are noticeably changed. Upon an increase in temperature, when crossover frequency fc comes closer to predetermined high frequency fH, the absolute value of .DELTA..epsilon.H approximates "0", and a response property with respect to the high-frequency electric field is considerably degraded. When fc exceeds fH, it is impossible to drive the liquid crystal by the two-frequency addressing scheme. When the temperature is decreased and crossover frequency fc is decreased, the absolute value of dielectric anisotropy .DELTA..epsilon.L in the low-frequency electric field is decreased, and the absolute value of dielectric anisotropy .DELTA..epsilon.H in the high-frequency electric field is increased. For this reason, response characteristics upon application of fL or fH are changed. In particular, if, .vertline..DELTA..epsilon.H.vertline. is large and/or if, .vertline..DELTA..epsilon.L.vertline. /.vertline..DELTA..epsilon.H.vertline. is small, the hysteresis effect caused by high-frequency electric field FH causes a large influence and considerably degrades response characteristics. If the influence of the high-frequency hysteresis effect noticeably appears, since a large force to align the liquid crystal molecules to be parallel to the substrates acts on the liquid crystal molecules, the liquid crystal molecules cannot be homeotropically aligned with respect to the substrates upon application of the low-frequency electric field within a short period of time. Therefore, light is incompletely shielded in the OFF state. If the ON state is to be obtained upon application of the high-frequency electric field, tilt alignment in the normal ON state cannot be obtained. The liquid crystal molecules are to be aligned in the homogeneous direction with respect to the substrates, so that the transmittance in the ON state is decreased. If this state continues, the liquid crystal molecules can no longer respond to the electric field at last.
As described above, the response characteristics of the conventional liquid crystal optical device considerably change upon changes in temperature, and an amount of light also a changes. Therefore, the conventional device has poor temperature stability, and has an insufficient amount of light.